計算機科学のブログ

name:                lesson
version:             0.1.0.0
github:              "githubuser/lesson"
license:             BSD3
author:              "Author name here"
maintainer:          "example@example.com"
copyright:           "2022 Author name here"

extra-source-files:
- README.md
- ChangeLog.md

# Metadata used when publishing your package
# synopsis:            Short description of your package
# category:            Web

# To avoid duplicated efforts in documentation and dealing with the
# complications of embedding Haddock markup inside cabal files, it is
# common to point users to the README.md file.
description:         Please see the README on GitHub at <https://github.com/githubuser/lesson#readme>

dependencies:
- base >= 4.7 && < 5
- containers

library:
  source-dirs: src

executables:
  lesson-exe:
    main:                Main.hs
    source-dirs:         app
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    dependencies:
    - lesson

tests:
  lesson-test:
    main:                Spec.hs
    source-dirs:         test
    ghc-options:
    - -threaded
    - -rtsopts
    - -with-rtsopts=-N
    dependencies:
    - lesson

module Main where

import qualified Data.Map as Map
import Lib
  ( RobotPart (..),
  )
import System.Environment (getArgs)

leftArm =
  RobotPart
    { cost = 2
    }

rightArm =
  RobotPart
    { cost = 3
    }

robotHead =
  RobotPart
    { cost = 4
    }

partsDB :: Map.Map Int RobotPart
partsDB =
  Map.fromList $
    zip
      [1, 2, 3]
      [leftArm, rightArm, robotHead]

main :: IO ()
main = do
  args <- getArgs
  let id = read $ head args
  let part = Map.lookup id partsDB
  print $ cost <$> part

module Lib
  ( RobotPart (..),
  )
where

data RobotPart = RobotPart
  { cost :: Double
  }

% stack exec lesson-exe  
lesson-exe: Prelude.head: empty list
% stack exec lesson-exe 0
Nothing
% stack exec lesson-exe 1
Just 2.0
% stack exec lesson-exe 2
Just 3.0
% stack exec lesson-exe 3
Just 4.0
% stack exec lesson-exe 4
Nothing
%